The present invention relates to the preparation and use of novel electrostatic discharge materials. More particularly, the invention relates to the preparation of high molecular weight, linear polyurethane-ureas from diisocyanates, polyoxyethylene diols and water and the use of these materials in applications in the electrostatic discharge areas such as a floor covering.
The present invention is also related to the use of a polyurethane-urea/polyethylene oxide (U-PEO) in manufacturing textile elements such as aprons and cots or roll covers. Textile aprons and cots or roll covers are used in drafting machines (such as spinning, drawing, roving and other drafting elements) for making yarns of cotton, rayon, worsted and synthetic fibers. The main function of cots (or roll covers) and aprons is to provide a surface to guide and control the roving to the spinning frame where "final draft" and twist are given to the fibers to convert them into useful yarns.
It has long been known that static charges which develop between isolated bodies are discharged when those bodies are brought into sufficiently close proximity or contact. Potentials as high as 30,000 volts have been reportedly generated simply by a person walking on a synthetic carpet. In recent years this phenomenon, always regarded as something of a nuisance, has become a major concern to the manufacturers of sensitive electronic equipment. A static discharge of only a few hundred volts can severely damage or ruin expensive electronic circuitry, and such damage can occur at any stage of the assembly process or during transportation or storage.
Static charges can accumulate on production workers, on assembly work surfaces, and on any of the tools and containers used in the assembly area. The need to prevent static discharge requires that the entire assembly environment be constructed from materials which will quickly dissipate static charge, effectively interconnecting all workers, surfaces and equipment with a common electrical ground.
There are two general classes of materials available for electrostatic discharge (ESD) protection in the electronics industries. The first of these classes is referred to as "conductive" materials, which typically have resistivities in the range of 10.sup.3 to 10.sup.6 ohms/square. These materials are typically made of plastics or elastomers which are filled or impregnated with conductive carbon black or metallic substances. In addition to their cost; most of these materials are dark and undecorative and cannot be coated for decoration, wearability or protection without losing their conductive qualities.
Conductive materials can also be made of inherently conductive polymers. Conductive polymers previously known include polyacetylene, polyphenylene and poly(pyrrole). Dopants such as AsF.sub.5, substituted quinones, FeCl.sub.3, HClO.sub.4, BF.sub.4 and iodine are added to improve conductivity. However, stability and compatibility problems as well as high cost and limited availability has limited the applications for these known polymers.
Charge dissipative materials, the second class, typically have resistivities in the range of 10.sup.6 to 10.sup.11 ohms/square. Most of these materials achieve their electrical conductivities through the use of topical chemicals such as antistats. These chemicals do not normally conduct electricity themselves, instead, they absorb moisture from the air which provides conductivity. Typically antistatic chemicals are small in molecular size. When mixed with plastics, they migrate to the surface and provide conductivity, particularly through moisture absorption.
For these reasons, the previously known charge dissipative materials are sensitive to the humidity of the environment. Furthermore, the performance of the material tends to degrade over time as the antistat which migrates to the surface might be lost by evaporation, cleaning or contact with other objects.
U.S. Pat. Nos. 3,852,255; 3,891,718; 3,893,979; and 3,911,047 disclose the synthesis of low molecular weight end-capped polyether polyurethanes for use as antistatic additives for fibers to provide surface resistivity values of the order of 10.sup.10 ohms after washing.
European Patent Application 0 142 792 discloses end-capped polyurethane oligomers useful for static dissipative coatings, providing 10.sup.12 -10.sup.13 ohms-cm volume resistivities.
U.S. Pat. No. 3,661,627 discloses using an aqueous bath to apply --CN groups to the surface of a preformed film or fiber to render it antistatic. U.S. Pat. No. 4,029,694 describes an ester/alkylene oxide antistatic agent for melt polymers such as the reaction product of dimethylterephthalate and an ethylene oxide adduct of a propylene oxide adduct of ethylenediamine. None of these references suggest the novel synthesis or polymers of the present invention.
In textile industries, the performance of cots and aprons used in the drafting system for spinning yarn is closely associated with the electrostatic characteristic of the composition of the cots and aprons. The quality of yarns generated from a drafting or spinning machine can be directly affected by the properties of the cots and aprons used in the system. In other words, when all variables in a yarn spinning process are kept constant, the drafting system with good cots and aprons will generate better quality yarns. The term "quality yarn" used in this description is a measure of the irregularity (i.e., the coefficient of variation CV) and the frequently-occurring yarn faults or "imperfections" of yarns generated from the drafting process. Such irregularities and "imperfections" of a yarn are directly related to the strength and uniformity of yarns with a given size. An actual physical measurement of the yarn quality can be performed using an Evenness Tester (e.g., the Uster Evenness Tester manufactured by Zellweger Uster Ltd. of Switzerland). The following parameters are generally recorded in a yarn quality test:
coefficient of variation--% CV. PA0 Number of imperfections per 1000 meters of yarn at a given setting of the imperfection indicator--i.e., number of thin places, number of thick places, and number of neps. PA0 resistant to lapping of fibers on the cot and apron surface (i.e., will not cause "ends down" and interrupt the yarn spinning process). PA0 resistant to generating clearer waste, i.e., a tendency for waste fiber pulled from the roving by the cot and accumulate on the clearer board in a bunch-like formation, this could also cause irregularity of the yarn product. PA0 resistant to abrasion caused by fibers passing through the surface of a cot (or an apron), i.e., long service life without rebuffing the cot surface or replacing the aprons to assure a good contact between fibers and the cot surface. PA0 resistant to viscoelastic creep relaxation, i.e., maintaining good dimensional stability without shape changing to assure consistent properties and uniform evenness of the final yarn product. PA0 (a) reacting a polyoxyethylene diol and an equivalent excess of an aliphatic diisocyanate in the presence of a tin catalyst to form a prepolymer; PA0 (b) mixing the prepolymer with water; and PA0 (c) heating to form a polymer. PA0 (1) preparing a linear polyurethane-urea/polyethylene oxide polymer; PA0 (2) mixing the polymer with another thermoprocessable polymer, and/or a filler.
It is generally believed that the hardness and surface characteristics (such as smoothness and electrical charge dissipation) of rubber based aprons and cots plays an important role in affecting the yarn quality in the yarn spinning process. Soft cots and flexible aprons are thought to be superior to hard cots and rigid aprons in terms of generating fewer irregularities and imperfections of yarns because soft cots and flexible aprons provide more surface area for contact between fibers and the cot and apron surface allowing a better control (less slipping) of the fibers in the yarn spinning process.
The electrostatic characteristic of a cot (or an apron) surface is important in a yarn spinning operation. Because yarn spinning processes are often carried out at high speed, the static charges, generated on the cot (or apron) surface by the contact between the high speed fibers and the cot (or apron) surfaces, pull fibers from the roving, causing imperfections and irregularities of the yarn products.
Considering the effect of cots and aprons on the efficiency of a yarn spinning operation and the quality of the final yarn product, a good cot and apron should possess the following characteristics:
As disclosed in the prior art, the lapping behavior of textile cots is connected to electrokinetic phenomena or the zeta potential (as measured by the electrical charges) at the cot surface (U.S. Pat. Nos. 2,450,409 and 2,450,410). Conventional cots were made of oil resistant synthetic rubbers, such as NBR (butadiene-acrylonitrile rubber). Such cot structures may be rendered lap resistant by the incorporation in the synthetic rubber of certain reaction products resulting from the digestion in water dispersion of a protein, such as the commercially available animal glue. U.S. Pat. No. 2,450,410 disclosed that cot structure containing animal glue and synthetic rubber could be made even better lap resistant if certain inorganic electrolytes or water ionizable inorganic substances were incorporated in the structure with the animal glue. U.S. Pat. No. 2,860,382 disclosed that polar plasticizers can also render antistatic characteristics to roll cover structures.
In a comparison of all of the ingredients used in a cot composition, animal glue is considered as one of the most important and one of the most widely used ingredients. In addition to rendering lap resistance, animal glue also reinforces a cot structure, providing resistance to abrasion and creep relaxation in actual cot performance. Unfortunately, the manufacturing process of textile cots involving animal glue is relatively labor intensive and time consuming. The mixing of animal glue with synthetic rubber requires fist digesting the glue in water, and after the glue is blended into the rubber, the water must be completely dried off before the vulcanization process. The water drying step is important because the water molecules remaining in the rubber composition will interfere with the rubber curing process and subsequently affect the performance of the final product. In addition, animal glue tends to harden the rubber cot structure, yet when the amount of animal glue used is reduced to make a soft cot structure, the final cot performance often suffers from lack of abrasion resistance and/or lapping resistance.